1. Field of the Invention
The present invention relates to a liquid crystal display (LCD), and more particularly, to a method for driving an LCD and an apparatus employing the same for improving picture quality.
2. Description of the Related Art
An LCD displays images by varying the arrangement of liquid crystal molecules by the action of an electric field to control light transmissivity. Types of LCDs that have been developed include the Twisted Nematic LCD (TN-LCD), the Super-Twisted Nematic (STN-LCD), the Metal-Insulator-Metal LCD (MIM-LCD) and the Thin-film Transistor (TFT-LCD), and LCD display performance has been remarkably enhanced. The LCD comes into the spotlight as an apparatus capable of replacing a CRT because it is compact and has a low power consumption. Demands for the LCD are increasing as the LCD is applied to a wide range of applications including portable TV, notebook computers, video phones, video cameras, mobile communication devices and so on.
The LCD includes an LCD panel in which pixels are arranged in an active matrix form, a gate driver and a data driver for driving the LCD panel. The LCD panel includes a color filter substrate and a thin film transistor array substrate, which are opposite to each other, and a liquid crystal layer formed of liquid crystal filled between the color filter substrate and the thin film transistor array substrate.
Common electrodes and pixel electrodes are respectively formed on the inner sides of the color filter substrate and the thin film transistor array substrate, which face each other. When a data signal is applied to the pixel electrodes while a common voltage is applied to the common electrodes and an electric field caused by a potential difference between a pixel voltage and the common voltage is applied to the liquid crystal layer. Accordingly, a desired image can be displayed by controlling light transmissivity of the liquid crystal layer by different data signals applied to the pixel electrodes.
Data lines for transmitting a data signal supplied from the data driver to the pixel electrodes and gate lines for transmitting a high gate voltage supplied from the gate driver to the pixel electrodes are formed on the thin film transistor array substrate. The data lines intersect the gate lines and the gate lines transmit the high gate voltage to the pixel electrodes such that the pixel electrodes are sequentially selected line by line.
Thin film transistors (TFTS) used as switching elements are respectively connected to the pixel electrodes. The TFTs are turned on by the high gate voltage supplied through the gate lines and the data signal provided through the data lines is applied to the pixel electrodes through the source and drain electrodes of the TFTs, and thus the light transmissivity of the liquid crystal layer is controlled by an electric field between the common voltage applied to the common electrodes and the data signal applied to the pixel electrodes.
In the LCD, however, controlling the arrangement of liquid crystal molecules accompanies a time delay and a response speed of the liquid crystal molecules is lower than a frame change rate because of unique characteristic of the liquid crystal molecules. This blurs the contour of a moving image or deteriorates picture quality when the moving image is displayed on the LCD.
To solve this problem, previous input data and current input data are compared to each other and the LCD panel is over-driven with maximum and minimum voltages of a source driver integrated circuit to increase the response speed of the liquid crystal. However, moving images are blurred because of hold type display characteristic of the LCD. Specifically, when a motion is generated on the screen of the LCD, the eyes of a viewer follow this motion. Here, the boundary of the motion appears blurry to the viewer because a hold type display such as an LCD maintains data written once for one frame.
FIGS. 1A and 1B are graphs showing motion blur generated in a conventional LCD driving method. A gray part in a white box represents a transitional stage in which one pixel is on or off as a frame is increased. The response speed of liquid crystal becomes higher as the gray part occupies a smaller area.
FIG. 1A illustrates a case that the response speed of liquid crystal is ½ frame. In this case, motion blur appears in 4.5 pixels. FIG. lB illustrates a case that the response speed of liquid crystal is 1 frame. In this case, motion blur appears in 6 pixels. As shown in the intensity graphs located at lower parts of FIGS. 1A and 1B, the edges have the same slope even when the response speed of liquid crystal is increased, and thus blurring of the edges cannot be prevented.
Accordingly, although the conventional LCD driving method can increase the response speed of the liquid crystal to reduce the motion blur, the edges remain blurred and the picture quality is deteriorated even when the response speed of the liquid crystal is increased.